1. Field of the Invention
The present invention relates to a base station, a mobile station and a method for data transmission in a communication system, in particular in a CDMA mobile radio system, where the data, structured in frames, is transmitted such that a mobile station is able to perform other functions; in particular, to carry out tests using a reception device, during one or more interruption phases in which it interrupts the reception (of the previous source or of the data from the base station) and/or the processing of received data or sending. “Transmission” is to be understood as sending and/or receiving below.
2. Description of the Prior Art
In communication systems, data (for example voice data, image data or system data) is transmitted on transmission links between base stations and mobile stations. In radio communication systems, this is done using electromagnetic waves via an air or radio interface. In this context, carrier frequencies are used which are situated in the frequency band provided for the respective system. In the case of the GSM (Global System for Mobile Communication), the carrier frequencies are in the range of 900 MHz. For future radio communication systems, for example the UMTS (Universal Mobile Telecommunication System) or other third generation systems, frequencies in the frequency band of 2000 MHz are provided.
Particularly in future CDMA systems, a base station basically sends continuously in the downlink direction, for example, that is to say in the direction from the base station to a mobile station. The data transmitted during sending is usually structured in frames which each have a prescribed length. Particularly with different services such as voice data transmission and video data transmission, the frames may also have different structures and lengths. The structure and/or length of each frame in a continuous series of frames is prescribed and/or is recognized by the mobile station, however.
Particularly in cellular mobile radio systems, the mobile station occasionally also needs to perform functions other than data reception which cannot be performed at the same time, at least during the operation of only a single reception device. By way of example, from time to time the mobile station in a radio communication system of cellular design in which the base stations in various cells send on different frequencies needs to test whether it is able to receive radio signals from another base station with good reception quality. For this purpose, the mobile station sets its reception device to a frequency other than the frequency on which it currently receives data.
In order to be able to send from the base station to the mobile station without interruption, it has already been proposed that the mobile station be equipped with a second reception device. In practice, however, this solution is usually rejected for cost reasons.
Another proposal is known according to which the base station interrupts sending at prescribed times in order to allow the receiving station to carry out an adjacent channel search (search for an adjacent base station or for particular data packets transmitted by these base stations, where data packets also may be understood below to be synchronization, frequency correction or pilot signal bursts) using its individual reception device.
To prevent a loss of data, the base station sends the data beforehand at a higher transmission rate than the fundamentally constant permanent transmission rate. So that this does not result in higher bit error rates (BER), the transmission power additionally needs to be increased during this time.
The frequency at which the interruption phases recur and the length of the interruption phases depend on the particular system and also on the particular operating state of the system. By way of example, interruption phases each having a length of 5 to 6 ms, respectively, are sufficient for an adjacent channel search by a mobile station in a radio communication system organized on a cellular basis. WO-A-97 25827 discloses a method for data transmission, in which interruption phases for observing an adjacent base station are inserted during transmission. However, provision is made in this case for the interruption phases to be inserted at regular intervals, which entails the disadvantage that the efficiency of the data transmission is reduced with each interruption phase.
Since the losses in transmission quality also increase with the number of interruption phases inserted, it is desirable for the smallest possible number of interruption phases to be inserted.
By way of example, a GSM frame transmitted by the GSM base station contains eight timeslots which each contain a data packet. The data packets transmitted by the GSM base station BS2, such as synchronization data packets (data packets to be detected, synchronization burst), frequency correction data packets (characteristic data packets, frequency correction burst) and normal data packets, are all subordinate to the same time frame. The GSM base stations transmit all 10 time frames (GSM frames) 4 times and, after a subsequent 11 time frames (GSM frames) (51 time frames in total), a frequency correction data packet, and one respective time frame later, a synchronization data packet.
If interruption phases based on the GSM standard were now inserted with a period of 26 time frames (GSM frames), the fact that the period of 51 time frames and the period of 26 time frames have no common denominator would cause a cyclic shift in the two time frame periods, so that after a maximum of 11 times 26 time frames, that is to say after 11 observation frames, the sought data packet which is to be detected would be received if the mobile station is not too distant from the respective adjacent base station BS2, BS3 or if no interference which is too intense arises during transmission.
It is an object of the present invention, therefore, to specify a method for data transmission, a mobile station and a base station which permit second base stations to be reliably observed while transmission quality is good.